Sintered electrical resistor



United States Patent SINTERED ELECTRICAL RESISTOR Gerard HeinrichJonlrer, Gerard Willem van Oosterhout, and Jan Heinrik van Santen,Eindhoven, Netherlands, assignors, by mesne assignments, to NorthAmerican Philips Company, Inc., New York, N. Y., a corporation ofDelaware No Drawing. Application June 27, 1955 Serial No. 518,388

Claims priority, application Netherlands June 26, 1954 4 Claims. (Cl.252-519) Sintered, electrical resistors on the basis of manganites withperovskite structure containing trivalent and tetravalent manganese areknown. These resistors exhibit a comparatively low temperaturecoefficient of for example 0.6% per degree centigrade and a low specificresistance of 0.10 cm.

There are furthermore described the semi-conductive properties ofcobaltites with perovskite structure, containing trivalent andtetravalent cobalt. Also with these materials the specific resistanceand the temperature coeflicient are low.

It has been found that on the basis of ferrites and chromites withperovskite structure semi-conductive resistors may be obtained, having ahigh temperature coeflicient at a comparatively low specific resistance.

The invention relates to a sintered, electrical resistor substantiallyconsisting of a phase of oxidic compounds with perovskite structure,containing Fe and/or Cr or else Co and Fe and/or Cr, in which at leastone of the metals Fe, Cr and Co occurs in the trivalent and thetetravalent form. In the conductive phase manganese in the tetravalentform may be included. In order to obtain a high temperature coefficient,it is desirable to restrict the Co content in the conductive phase tonot more than 90 atom percent, calculated on the total content of Fe,Cr, Co and Mn.

Part of the said metal ions may furthermore be replaced by other smalltrivalent or tetravalent metal ions, for example of Al, Ti and Ni. Inorder to prevent the specific resistance from becoming too high, it isdesirable to restrict its content to not more than 20 atom percent.

The general formula for the chemical composition of perovskites is ABOwherein A designates a large metal ion, having an ion radius accordingto Goldschmidt of more than 1.0 A. and B a small metal ion having an ionradius between 0.5 and 1.0 A., the sum of the valencies of A and B being6. Both A and B may designate a mixture of metal ions which fulfil theafore said conditions.

It is known that the obtainment of conductivity in heteropolar crystalsrequires that they should contain one metal ion occurring in differentvalencies. This is permitted by the metal ions Fe, Cr and Co, which mayoccur in the perovskites in the trivalent and the tetravalent form, if amixture of trivalent and bivalent metal ions is chosen for the largemetal ions designated by A in the general formula. To this end use maybe made of the trivalent metals yttrium and the so-called largelanthanides, for example La, Pr and Nd and the bivalent metals forexample Ca, Sr, Ba and also Pb and Cd.

, 2,862,891 Patented Dec. 2, 1958 The latter two are less suitablebecause of the volatility of their oxides.

A mixed crystal of LaFe +O with SrFe +O inay, for example, be formed bysintering in air La O SrO (if desired in the form of carbonate) and Fe Oat temperatures lying between 1250" C. and 1400 C. In this case thedesired concentrations of Fe and Fe are obtained by a choice of theratio between La and Sr.

In general with the sintering in air the ratio between theconcentrations of the trivalent and the tetravalent small metal ions,may be adjusted fairly accurately in accordance with the formula ABO; bymeans of the concentrations of the trivalent and the bivalent largemetal ions, if the latter ratio exceeds about 1.5.

At a higher content of large bivalent metal ions not all oxygenpositions in the perovskite lattice areoccupied so that the content oftetravalent small ions is smaller. The oxygen content may be acted uponto some extent by means of the partial 'oxygen pressure of the gaseousatmosphere in which the sintering takes place.

If the A-position in the perovskite is completely occupied by largebivalent ions, the deviation from the 1 formula ABO may be considerable.For strontium ferrite, sintered in air at 1320 C. the composition SrFeOhas been found. Since the effect of the partial oxygen pressure iscomparatively slight, stable resisters are also obtained with theperovskites having an oxygen deficiency.

It has been found that perovskite compositions containing Fe or Crexhibit a high temperature coefficient at a comparatively low specificresistance. A greater variety in resistance properties may be obtainedby combining the Feand Cr-containing perovskites with one another orwith the other aforesaid metals.

In order to obtain a favourable temperature coefiicient it is necessarythat the sintered products according to the invention should beconstituted mainly by the aforesaid conductive phases. Moreover, otherconductive or nonconductive phases may be contained in the sinteredproducts to a content of about 15% by volume. If desired, fluxes, forexample copper oxide, zinc oxide, clay and glass may be added to aquantity of about 10% by weight.

The products according to the invention may be obtained from thecomposing oxides, if desired in the form of double oxides or compounds,for example carbonates, changing into oxides during sintering. The saidconstituents are ground, mixed, worked up to obtain a paste, for examplewith an organic binder, and then, subsequent to shaping, sintered in anoxygen containing atmosphere. The constituents may, as an alternative,be caused to react, subsequent to mixing, at a higher temperature: theymay then be ground again and sintered subsequent to shaping.

The invention will be illustrated with reference to the examplesindicated in the following table. This table indicates a fewcompositions of the sintered products with the corresponding specificresistances p in 9 cm. at room temperature, the activation energy qbeing indicated in e. v., which is a measure for the temperaturecoefficient in accordance with the formula L p=pand the temperaturecoefiicient itself in percent per degree centigrade at room temperature.

Ca Sr Pb) (Fe 99999999999 8&3838583838 llllllillillllllllillllllll9999999999999999999999 +27 by weight 2110 +27; by weight T:

MafiNNCRW 9999999999999999999999999999999999999 823255855E85$585GG$5585E885583$88882555 l I I I l I l I I 99999999999999!- What isclaimed is:

l. A sintered electrical resistor having a conductive phase consistingof crystals having a perovskite structure and a compositioncorresponding approximately to the formula AB D ,,O wherein A representsa mixture of at least one trivalent metal ion selected from the groupconsisting of La+++, Nd and Pr+++ and at least one bivalent metal ionselected from the group consisting of Ca++, Sr++, Ba++ and Pb++; Brepresents at least one trivalent metal ion selected from the groupconsisting of Fe+++, Cr+++, and Co+++, D represents at least onetetravalent metal ion selected from the group consisting of Fe++++cr++++ Co++++ and Mn++++; Mn++++ Co+++ and Co++++ being present onlywhen at least one of the other metal ions other than the metal ionsrepresented by A in said formula is present, the elements Co, Fe and Crbeing present in both the trivalent and tetravalent states, and whereinn represents a numeral having a value greater than zero but less thanone.

2. The resistor of claim 1 in which the conductive phase contains Co inan amount up to atom percent of the total content of Fe, Cr, Co and Mn.

3. The resistor of claim 1 wherein the conductive phase contains inaddition at least one metal selected from the group consisting of Al, Tiand Ni in an amount up to 20 atom percent of the total content of Fe,Cr, Co and Mn.

4. The resistor of claim 1 wherein in addition up to 10 percent of aflux is present.

References Cited in the file of this patent Physica, v. 16, No. 3, March1950, pages 337-349. Physica, v. 19, 1953, pages -123.

1. A SINSTERED ELECTRICAL RESISTOR HAVING A CONDUCTIVE PHASE CONSISTINGOF CRYSTALS HAVING A PEROVSKITE STRUCTURE AND A COMPOSITIONCORRESPONDING APPROXIMATELY TO THE FORMULA ABND1-N03 WHEREIN AREPRESENTS A MIXTURE OF AT LEAST ONE TRIVALENT METAL ION SELECTED FROMTHE GROUP CONSISTING OF LA+++, ND+++ AND PR+++ AND AT LEAST ONE BIVALENTMETAL ION SELECTED FROM THE GROUP CONSISTING OF CA++, SR++, BA++ ANDPB++; B REPRESENTS AT LEAST ONE TRIVALENT METAL ION SELECTED FROM THEGROUP CONSISTING OF FE+++, CR+++, AND CO+++, D REPRESENTS AT LEAST ONETETRAVALENT METAL ION SELECTED FROM THE GROUP CONSISTING OF FE++++,CR++++, CO++++ AND MN++++; MN++++, CO+++ AND CO++++ BEING PRESENT ONLYWHEN AT LEAST ONE OF THE OTHER METAL IONS OTHER THAN THE METAL IONSREPRESENTED BY A IN SAID FORMULA IS PRESENT, THE ELEMENTS CO. FE AND CRBEING PRESENT IN BOTH THE TRIVALENT AND TETRAVALENT STATES, AND WHEREINN REPRESENTS A NUMERAL HAVING A VALUE GREATER THAN ZERO BUT LESS THANONE.